A pneumatic weighing device is disclosed in U.S. Pat. No. 4,306,629, wherein an air cushion is, employed to support an article passing through a weighing station. Weight of an article is detected by employing a pressure transducer to measure the amount of pressure required to support the article.
The present invention is directed towards a weighing mechanism, wherein a weigh pan is provided with a plenum chamber having an apertured upper surface through which air under pressure is directed for purposes of providing an air cushion for supporting and transporting articles to be weighed. The weigh pan is supported on a conventional load cell for measuring the weight of the article supported above the weigh pan by the air cushion. Articles are transported onto and from the weigh pan by vertically stationary air cushion devices.
The weighing mechanism additionally includes pneumatic means for rejecting articles of improper weight from a stream of articles passing downstream from the weigh pan.
In an alternative embodiment, an apertured vacuum chamber is supported for vertical movement with the weigh pan and provides a zone of reduced pressure cooperating with the air cushion to support irregularly shaped articles in spaced relation to the apertured surface of the weigh pan.
In a further alternative embodiment, an apertured vacuum chamber is connected directly to a load cell in the absence of the provision of an air cushion above a weigh pan for use in weighing articles having upwardly-facing surfaces, which are relatively large compared to their downwardly-facing surfaces.
The present invention permits an increase in accuracy of the weighing operation, as compared to prior known weighing mechanisms employing belt or chain conveyors passing across a load cell mounted weigh pan, and provides for a lower cost installation. Moreover, it is contemplated that the invention will allow for substantial increases in the numbers of articles weighed per unit time and/or the speed at which articles are transported through a weighing station.